The issue here is that Dallas went with "Luka's conditioning is bad, we don't want to sign him, etc." Once you do that, you devalue your own player and tank your own leverage. If you open the trade up to other teams, by definition you are saying you don't want the player, and people would have found out why.
Using this list is helpful:
By opening up the trade to other teams, by definition you are moving Luka down a few spots given the issues I mentioned with leverage. In that scenario, getting Davis, who was #9 in this trade value list, and the third big man behind Jokic and Giannis, may actually have been the best they could have done in terms of actual players. Other teams may have been able to offer more young players / draft picks